The example in the documentation shows not only copying between filesystems, but also renaming at the same time.  In such a situation, I would expect to need to provide the new filename.

The example I have provided works as I expected it to when using Databricks Runtime 10.4LTS, but works the way I have described it when using DBR 13.3LTS.  Thus, it is either a new bug, or an expected change in behavior or fix of a previous bug that should be documented.